An electromagnetic field, and indeed a single photon, carries energy and momentum. The momentum may comprise two components. Linear momentum is responsible for the observable radiation pressure, whilst angular momentum is the rotation of the wave around its own axis as it propagates forward and is observable as a radiation torque. The angular momentum also comprises two components, namely spin angular momentum and orbital angular momentum (OAM). For a paraxial beam spin angular momentum is associated with polarization, whilst OAM is associated with the spatial field distribution.
Of particular interest is the origin independent internal OAM, which can be associated with a helical wavefront shape. In these helical modes the electromagnetic field has a helical wavefront shape with a central vortex such that the beam phase varies in a corkscrew-like manner in the beam propagation direction.
The OAM carried in such a field enables it to trap and rotate colloid particles and living cells as a so called “optical spanner” for use in biophysics, micromechanics or microfluidics. OAM also has the potential to be used in super-high optical data storage, imaging and metrology, or in free-space communications. More generally OAM has great potential for new and wide-ranging applications in both classic and quantum optics.
Current techniques for generating light carrying OAM using bulk optics, including computer generated holograms, spiral phase plates, q-plates and dove prisms, have limitations in terms of efficiency, cost, flexibility and scalability. These methods do not lend themselves to integration which is essential for widespread and large scale utilization in future applications.